This invention relates to the use of Light Detection and Ranging (LIDAR) to detect elements in the atmosphere remotely. More particularly, this invention relates to mobile use of modulated tunable diode lasers in order to sweep the laser wavelength through an absorption line of a gas such as methane in order to determine the presence of the gas in the atmosphere.
LIDAR systems operate somewhat like radar. LIDAR, however, directs laser light rather than radar waves at a particular target to detect the target. The laser light may be pulsed or relatively continuously generated, and it may be focused or collimated as desired to reach the desired end. Objects, particles, and gases can scatter and/or absorb the laser light. Thus, the measurement of the reflected light can provide information about the target or atmospheric constituents along the optical path. LIDAR data is derived by collecting the scattered (reflected) light with a telescope, which focuses the collected light onto a photodetector. The amount or intensity of the light thus detected can be processed to provide information about the object being scanned and the optical path through which the laser beam passes.
LIDAR systems have been used in the past for the mobile determination of the presence of particular gases, such as the presence of methane from a gas line leak, in the atmosphere. One such LIDAR system employs an Optical Parametric Oscillator (OPO) as the laser source. OPO based LIDAR is particularly effective for determining the presence of methane because the wavelength of the OPO-based light lies in the fundamental absorption band of the methane gas.
On the other hand, OPO-based LIDAR is expensive and also requires extreme environmental controls to maintain stable long term operation. OPO systems are complex and prone to alignment problems, requiring highly trained maintenance personnel.
Also, since OPO-based LIDAR emits pulses of laser light, the pulse repetition frequency (PRF) can present a significant problem for mobile applications seeking to detect small gas plumes, such as gas leak plumes. Most currently used commercial OPO-based systems having sufficient output energy to detect such plumes operate at a PRF of 10 Hz or less. At such extremely low pulse repetition rates, the speed of the mobile measurement platform can strongly influence the measured data. The mobile platform is thus not only likely to miss some plumes entirely but also can incorrectly estimate plume concentrations as the OPO is tuned between wavelengths and the target moves relative to the OPO-based system. Although the latter, moving-target problem can be reduced by using two OPO-based LIDARs that near simultaneously transmit differential wavelength pairs, this dual-OPO laser system is not only expensive but also very complex and does not solve the former, low PRF problem.
Recently, OPO-based systems have been developed that provide higher PRF rates (in the kHz range). One such system is that developed by Sandia National Laboratory. These systems, however, produce micro-joule energies due to the high PRF, requiring long integration times to accomplish detection. For this reason, the system will likely miss small or low concentration plumes, particularly in the mobile environment. These systems are also very expensivexe2x80x94probably too much so for use by pipeline survey companiesxe2x80x94and they are difficult to maintain in alignment, especially in a mobile application. This is because OPO-based systems require extreme environmental controls and stability to operate properly. Field and mobile applications generally do not allow for these types of controls.
Another prior art LIDAR technique uses frequency mixing to generate emissions in the fundamental absorption band of methane. These frequency-mixing systems use expensive lasers (such as ND-YAG and Ti:Sapphire lasers in downconverting frequency mixing schemes or CO2 lasers in upconverting devices). Like the OPO-based systems, they also are non-linear crystal-based systems that are difficult to maintain in alignment, especially in mobile applications.
There are also Tunable Diode Lasers (TDLs) that have been developed for the detection of methane gas plumes in the atmosphere. One such TDL laser has been developed by the Tokyo Gas Company. The Tokyo Gas TDL laser is reported to have sufficient sensitivity to detect gas line leaks, using low frequency wavelength modulation and lock-in (phase differential) detection. Low frequency lock-in detection, however, has several major disadvantages for mobile, remote detection operations.
First, low frequency lock-in detection requires long scanning and data averaging times to achieve sufficient sensitivity to detect small remote plumes. As a result, low frequency lock-in detection TDL LIDAR techniques are effectively limited to static line-of-sight, not mobile, applications.
Second, although there are other processing techniques such as matched filtering that can often be used in LIDAR systems to improve sensitivity, these techniques cannot be used with low frequency TDL LIDAR systems. This is because these types of processing techniques are based on the absorption line signature information which require use of much higher (RF) frequencies.
While there are lasers available, such as the OPO-based LIDARs described above, that operate within the fundamental absorption level and overtone band of gases such as methane, the applicants believe that such systems have not provided a solution to the problem of using LIDAR to economically and reliably detect gas leaks, particularly methane gas leaks, in mobile applications.
There have been TDL-based lasers in the prior art that operate in the first overtone band, but not in the fundamental absorption band, of gases such as methane, but they have not been applied to mobile detection of gases such as methane. Because such lasers operate in only the overtone band, they are not as readily absorbed by gases such as methane. Applicants believe that, as a result of this limitation and possibly other aspects of TDL-based lasers, such lasers have not been applied to the mobile detection of gases such as methane.
Frequency Modulation Spectroscopy (FMS) techniques exist in the prior art, such as those identified in U.S. Pat. No. 4,594,511 (xe2x80x9cthe ""511 patentxe2x80x9d), entitled xe2x80x9cMethod and Apparatus for Double Modulation Spectroscopy,xe2x80x9d issued to one of the present inventors, and in U.S. Pat. No. 5,572,031 (xe2x80x9cthe ""031 patentxe2x80x9d), entitled Pressure and Temperature Compensating Oxygen Sensor, issued to two of the present inventors.
As the ""511 Patent explains, FMS can be used to detect spectral properties of a sample more economically, conveniently, and accurately than detection techniques operating strictly in the frequency domain of the information of spectroscopic interest. The ""511 Patent also states that such FMS techniques can be used to take measurements of gaseous samples.
Although the ""511 patent does suggest that FMS techniques may be used with a variety of lasers including TDL-based lasers, the ""511 patent does not teach how to apply FMS techniques to any particular TDL apparatus. The ""511 patent also does not teach any mobile apparatus or method or use of FMS or TDL techniques to detect methane gas in particular, much less remotely detect methane gas in the atmosphere.
The applicants have invented a method and apparatus for remote detection of gas, preferably methane, dispersed into the atmosphere. The method utilizes a TDL-based LIDAR, utilizing a TDL whose frequency can be altered by changing the TDL drive current. The TDL laser is driven by a drive current or carrier, and the carrier frequency is preferably centered in the center of the absorption line of the gas in issue. A small RF modulation current (preferably at 4 MHZ) is superimposed on the TDL carrier frequency to produce sidebands, which lie within the pressure broadened absorption line of the gas. A low frequency (about 1 KHz) sawtooth ramp current is also superimposed on the TDL drive current to sweep the carrier and its associated sidebands over a range, and the range preferably is twice as wide as, and centered on, the atmosphere-pressure broadened absorption line width of the gas. The resulting TDL light is directed at an uncooperative target and collected by a detector. An uncooperative target is one which is undefined, such as a methane plume, rather than a defined target such as a retroreflector. The collected sideband laser light and carrier signal are then fed to an FMS processor to generate a derivative signature, preferably a second derivative signature (derived from the mixing of upper and lower sidebands) that indicates whether the gas is present in the atmosphere. A closely related technique, WMS, or wavelength modulation spectroscopy, can achieve comparable results in certain situations and may be used in some instances in place of FMS in the present invention.
The present method and apparatus preferably includes a reference WMS gas detection technique. The reference provides a baseline for comparison of the atmospheric derivative signature with the reference derivative signature and confirmation that any apparent detection of the gas in issue from the atmosphere derivative signature is consistent with the reference signature and not likely to be the result of misalignment, anomalous performance of the apparatus, or gas other than sought to be detected in the atmosphere.
It is to be understood that this is a brief summary of aspects of the invention. There are other aspects of the invention that will become apparent as the specification proceeds.
It is therefore an object and an advantage of the present invention to provide an apparatus and method for mobile detection of gas leaks in the atmosphere, particularly methane gas leaks.
It is an advantage of the present invention to provide a LIDAR (and method of using it) that is relatively sensitive, mobile, and economical.
It is yet another advantage of the present invention to provide a LIDAR that is relatively stable and rugged, and can operate unattended or with relatively minimal attention by an operator.
It is a further advantage of the present invention to provide a relatively compact LIDAR for detection of gas plumes.
It is a still further advantage of the present invention to incorporate commercially available TDL""s to achieve the mobile detection of gas plumes in the atmosphere.
It is an additional advantage of the present invention to provide a mobile gas detection LIDAR that utilizes a single source emitter.
Another advantage is that the present invention provides continuous wave operation, reducing the likelihood of missing the detection of a remote gas line leak when the apparatus is moving.
It is yet another advantage of the present invention to provide a gas detection technique that not only is mobile but also provides for self-calibration through the gas detection process.
A further advantage is that the present invention includes a reference gas detection signature to compare against an apparent detection of gas in the atmosphere and ensure that the apparent detection is correct.
A still further advantage of the invention is that it can use an TDL laser that emits light in other than the primary absorption band of the gas under study and yet detect the gas.
A still further advantage of the invention is that it can use an TDL laser in conjunction with a master oscillator/fiber amplifier transmitter that has no moving or adjustable parts at all. An all-solid-state monolithic and integrated device can be achieved, which leads to a compact and virtually maintenance-free LIDAR system.
There are other objects and advantages of the present invention. They will become apparent as the specification proceeds.
In this regard, it is to be understood that, although the applicants"" believe that their preferred embodiment described herein meets the objects and provide the advantages recited herein, the scope of the invention is to be determined by reference to the claims and not necessarily by whether any given embodiment achieves all objects and advantages stated herein.